Department of Defense
(DoD) "The Department of Defense (DoD Directive 5100.1) is responsible for providing the military forces needed to deter war and protect the security of the United States. The major elements of these forces are the Army, Navy, Air Force, and Marine Corps. Under the President, who is also Commander-in-Chief, the Secretary of Defense exercises authority, direction, and control over the Department which includes the Office of the Secretary of Defense, the Chairman of the Joint Chiefs of Staff, three Military Departments, nine Unified Combatant Commands, the DoD Inspector General, fifteen Defense Agencies, and seven DoD Field Activities."

Source:
DoD Organization & Functions Guidebook